This invention relates generally to communications systems and, more particularly, to the reception of communication signals containing pseudorandom code sequences. A pseudorandom sequence is a sequence produced by some definite arithmetic process, but satisfying one or more of the standard tests for randomness. As used in communications, a pseudorandom code sequence is a sequence of digital, and typically binary, numbers, that repeats itself after a time, but is long enough that the sequence can be considered random for most purposes. Pseudorandom code sequences are sometimes referred to as pseudorandom noise signals, or pseudo noise signals, or PN signals. For brevity, the term PN signals will be used throughout this specification.
PN signals are used in a number of different applications, including spread spectrum communications, distance ranging equipment, synchronization of burst communications systems, navigation systems using orbiting satellite, such as the global positioning system (GPS). What all these applications have in common is that they have the capability to detect a known PN signal in the presence of noise and to estimate the arrival time of the signal at a receiver. It may generally be assumed that the receiver has no knowledge of the phase of the carrier signal on which the PN signals are modulated. This is said to be "incoherent" demodulation, as distinguished from "coherent" demodulation, in which the frequency and phase of the received carrier signal must be precisely established. Further, although PN signals may take various forms, the present invention is concerned with PN signals that are bi-phase modulated antipodal signals of constant energy. That is to say, the carrier signal is modulated by switching its phase between zero and 180 degrees to encode a binary pseudorandom sequence.
There are slightly different performance requirements for specific applications, but in general each application involving PN signals will have two functional requirements. First there is a requirement to know when the received signal time phase is within some time window of the local receiver time phase, this requirement being referred to as signal detection. Second, if the signal is detected, there is a requirement to determine whether the time of arrival of the signal is "early" or "late" compared with some reference time, and preferably also provide a quantitative measure of the earliness or lateness of the signal. The early/late measurement and signal detection information can then be used by either a transmitter or a receiver to adjust its time phase for synchronization.
The problem of detecting a known signal in the presence of noise, and estimating its time of arrival, without a knowledge of the carrier phase, is a classic one in the radar field, and various solutions have been developed. In general minimum required signal-to-noise ratio (SNR) is considered the optimal solution to the problem.
All the receiver architectures discussed in this specification use correlation for detection and synchronization of the received PN signal. The received signal is correlated non-coherently with a local representation of the transmitted signal. This is a complex correlation and, since phase is unknown, only magnitude information is retained in the correlation. This magnitude is then compared with a threshold level that is appropriately selected to provide a probability of detection of the signal.
To determine if the signal is early or late, it is correlated to both an early and a late local signal, the difference between the two correlation magnitudes providing a measure of time error in the received signal. The classic circuit architecture for handling this includes two parallel correlation paths, one for the early correlation and the other for the late correlation. Signal detection is effected by summing the early and late correlation magnitudes and comparing the result to a threshold. Alternatively, the early and late correlation magnitudes can be separately compared to thresholds, with detection being declared if either comparison yields a magnitude over the threshold. This process is analogous to a delay lock loop used in spread spectrum systems.
An alternative to the parallel correlation process of the delay lock approach is to time-share the correlation hardware between the two required correlations of early and late signals. Time-sharing the receiver hardware to correlate the received signal with an early reference half of the time, and a late reference the rest of the time, reduces the complexity and cost of the hardware, but at a cost of approximately 3 dB in SNR performance. Intuitively, it can be seen that performance will be degraded by a factor of two, because each correlation path is receiving data for only half of the time, as compared with a circuit using two sets of correlation hardware in parallel correlation paths. The time-sharing correlation circuit is analogous to the tau-dither approach in spread spectrum systems. Clearly, there is a design trade-off between cost and performance. Better SNR performance is obtained from the delay lock approach, but at a cost of complexity. A lower cost is obtained from the tau-dither approach, but this results in degraded SNR performance. The present invention eliminates this difficult design choice and provides the optimal SNR performance of the delay lock circuit, but with a cost and complexity similar to the tau-dither circuit.